310 VETERINARY PHYSIOLOGY 



when it is increased by section of the splanchnic nerves, they 

 are more marked. But they persist after section of both 

 nerves, and they must therefore be presided over by the 

 local nervous mechanism in the wall of the stomach. They 

 are inhibited by taking food and even by the sight or smell 

 of food. They do not stop during sleep. 



B. Stomach after Feeding. 



When food is taken, (1) the blood-vessels dilate, (2) a 

 secretion is poured out, and (3) movements of the organ 

 become more marked. 



1. Vascular Changes. — The arterioles dilate, and the 

 mucous membrane becomes bright red in colour. This is a 

 reflex vaso-dilator effect, impulses passing up the vagus to 

 a vaso-dilator centre in the medulla, and coming down the 

 vagus from that centre. Section of the vagi is said to pre- 

 vent its onset. 



2. Secretion. — There is a free flow of gastric juice from 

 all the glands in the mucous membrane. 



(1) Characters of Gastric Secretion. — The gastric juice from 

 the cardiac end is a clear watery fluid, whicli is markedly 

 acid from the presence of free hydrochloric acid. In the dog 

 the free acid may amount to over 0'4 per cent., but in the pig 

 it is less abundant, and, when the gastric juice is mixed with 

 food, the acid rapidly combines with alkalies and with pro- 

 teins and is no longer free. In addition to the HCl, small 

 quantities of inorganic salts are present. Traces of proteins 

 may also be demonstrated, and two enzymes are associated 

 with these — one a proteolytic or protein-digesting enzyme, 

 'pepsin, the other a milk-curdling enzyme, reniiin. The fact 

 that dilution may abolish the peptic activity while leaving 

 the curdling action intact seems to show that these are not 

 separate bodies but phases in the activity of one body. 



The secretion from the pyloric portion is alkaline in 

 reaction. 



(2) Source of the Constituents of the Gastric Juice. — The 

 hydrochloric acid is formed at the cardiac part of the 

 stomach. This may be shown by making a small stomach 

 by the method of Pavlov. Since the parietal or ox^'ntic cells 



